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The earliest images of 3I/ATLAS, newly uncovered by Michigan State University, reveal how the interstellar object evolved as it traveled through our solar system—and how other distant solar systems might be different from our own.

Even before the comet was discovered in early July, it was hiding among the stars, too faint to be noticeable. In a new paper in the Astrophysical Journal Letters, MSU researchers combed through images from NASA's Transiting Exoplanet Survey Satellite, or TESS, and found evidence of 3I/ATLAS as early as May.

The images suggest that the object may have been active for longer than scientists expected, with clues hidden in its comet-like tail. They also provide scientists with more information about an object that will only be visible in the sky for a few more months before it zooms beyond even the most powerful telescope's reach.

"There are so few interstellar objects that we've detected in our solar system, and they each seem to be unique," said Adina Feinstein, assistant professor of physics and astronomy at MSU and lead author of the study. "3I/ATLAS gives us an opportunity to study other solar systems up close and personal, without actually needing to visit them."

MSU published the first scientific paper just days after 3I/ATLAS was spotted. It's dubbed "3I" because it's only the third interstellar object detected so far, and ATLAS comes from the telescope network that identified it—NASA's Asteroid Terrestrial-impact Last Alert System.

Since then, scientists around the world have rushed to observe and document as much as they can about the object before it's gone. Feinstein said papers are published nearly daily as the astrophysics community works at a frenzied pace.

Interstellar objects aren't Feinstein's usual field of study, but she uses TESS data to look for distant planets that pass in front of their stars. Occasionally, TESS observes the plane where our solar system's planets are aligned—the very plane where 3I/ATLAS was traveling. Feinstein challenged herself to use this data to contribute to research that has captured global imaginations.

"As soon as I realized 3I/ATLAS should be in these TESS images, I downloaded them as quickly as I could," Feinstein said.

After filtering out light spillover from nearby stars, the team combined multiple images taken at different points across 3I/ATLAS's orbit to create a sharper picture. This technique made the interstellar object pop clearly amid a sea of pixels.

The additional two months of images told a story of a solar system very different from our own. In our system, comets that approach the sun release dust and gas—mostly water—from their surface as the temperature increases. But photos showed activity from 3I/ATLAS farther from the sun than researchers expected. That means it's likely releasing other molecules, like carbon monoxide and carbon dioxide.

"That gives us an idea of what comets and asteroids could be made of in systems beyond our own sun," Feinstein said. "It's always been assumed that other systems are different from ours, but now we have evidence of how different they might be."

Researchers expect 3I/ATLAS to be visible for several more months. In the meantime, MSU's research team will continue to study and learn as much as they can about the interstellar interloper, including Assistant Professor Darryl Seligman, a co-author on the paper who also wrote the first 3I/ATLAS paper.

They'll also use observations from NASA's James Webb Space Telescope to learn more about what 3I/ATLAS is made of.

"Capturing as many observations from this prediscovery period, where 3I/ATLAS may be in telescope images but wasn't previously identified, is essential to our understanding of how these objects 'turn on' as they approach our sun," said John Noonan, co-author and postdoctoral fellow at Auburn University. "These interstellar objects have likely not been warmed significantly in millions, if not billions, of years, and any opportunity to see how they responded to that early heating is of interest."

The new research highlights why all-sky surveys are crucial. Missions like NASA's TESS were designed to observe stars and potential planets, but they also provide huge datasets that can help astrophysicists solve mysteries about comets, asteroids and interstellar objects. Federal funding is the backbone of what keeps these critical programs going, Feinstein said.

"Learning about other solar systems places humanity into context," Feinstein said. "One of life's greatest questions is 'are we alone in the universe?' Each NASA mission gets us a little bit closer to answering this big, overarching question."